1. Field of the Invention
The present invention relates to the preparation of a synthetic faujasite zeolite. More specifically, the present invention relates to a method of producing highly crystalline, defect free, hydrothermally stable zeolite which has a crystallite size of 1500-2000 .ANG. and which is suitable for gas oil cracking.
2. Description of the Related Art
Synthetic faujasite zeolites or crystalline aluminco-silicate zeolites commonly known as molecular sieves are characterised by a very highly ordered crystalline structure. This involves a three dimensional framework of AlO.sub.4 and SiO.sub.4 tetrahedra which are cross linked by the sharing of oxygen atoms. The electrovalency of each tetrahedron containing aluminum is balanced by the presence of a cation such as an alkali metal ion. The void spaces in the framework are occupied by water molecules. The chemical formula for a faujasite zeolite given in U.S. Pat. No. 3,130,007 is as follows: 0.9 +/-0.2 Na.sub.2 O: Al.sub.2 O.sub.3 : w SiO.sub.2 : x H.sub.2 O, where w has a value of greater that 2.5 and up to 6, and x may have a value as high as 9. Synthetic faujasite zeolite, also known as Sodium Y zeolite, has been in use as a key component of a Fluid Catalyst Cracking (FCC) catalyst for over past 30 years. Attempts have been made to synthesise a large pore zeolite to replace the Sodium Y zeolite for a FCC catalyst, but these have failed to meet stringent hydrothermal stability requirements. Efforts to improve hydrothermal stability of Y zeolites have led to the development of ultrastable Y zeolite (USY), but the process of ultrastabilisation is expensive. The other drawbacks of USY zeolites are that nonframework alumina produces undesirable coke and gas, ultrastabilisation by diaminohexafluorosilicate causes a reagent effluent problem and finally USY zeolite have aluminium depleted surfaces resulting in a loss catalytic activity.
In the FCC process, in which the feed contains a wide range of molecular sizes and types, some reactions will take place on the surface and others inside the zeolite crystal. In the prior art, attention has been focused to increasing surface to volume ratio of a Y zeolite by reducing crystal sizes by manipulation of synthesis conditions. European patent NO. EP 02423GA2 describes preparation of a type zeolite having smaller than conventional particle size which, when formulated into a FCC catalyst exhibits improved product selectivity. Since most samples of zeolite are agglomerates with a rock pile morphology, crystallite size is not the same as particle size. Particle size and crystallite size are the same only if atoms are stacked linearly in all three dimensions continuing up to the crystal surface. It is well known from various publications that smaller sized zeolite crystals have abnormally rapid deactivation rate in the presence of steam probably reflecting the preferential dealuminaticin of the surface or destruction of the crystal lattice by high rates of surface hydrolysis.
This problem was addressed in the prior art by increasing the Si/Al ratio through ultrastabilistation which, in itself, is an expensive process. Defects such as stacking faults, screw defects, presence of impurity, and amorphous material in the zeolite are the major causes for their poor hydrothermal stability. Grain size or crystallite size has a direct correspondence to stacking faults. Two crystallities or grains are separated by a boundary called a grain boundary where linearity of atomic arrangement is interrupted.
Grain boundaries are considered basically as structrual defects, along which some atomic planes get terminated. At the line of termination, covalent bonds between atoms are strained. At this point, lattice atoms can be easily removed at high temperature in the presence of steam or acid, thereby losing a significant amount of crystallinity. With decreased average crystallite size, total length of the grain boundary increases, as shown in FIG. 1 of the accompanying drawings, which makes the crystals more fragile.
An object of this invention is to propose a process for the preparation of synthetic faujasite zeolite having a larger crystallite or grain size in comparison to that of the known art.
Still another object of this invention is to propose a process for the preparation of synthetic faujasite zeolite having a reduced total length of grain boundary.
Yet another object of this invention is to propose a process for the preparation of synthetic faujasite zeolite which exhibits improved hydrothermal stability, activity and metal tolerance.